As is well known in the art, it is required to reduce the pattern rule so as to comply with the recent demand for higher integration densities and operating speeds in LSI devices. Accordingly the exposure method and resist composition have noticeably changed. Particularly in the lithography process of forming patterns with a feature size of 0.2 μm or less, the exposure light is switched to KrF or ArF excimer laser radiation or electron beam, and the photoresist composition is changed to a chemically amplified resist composition having a good sensitivity to such high-energy radiation and a high resolution.
Resist compositions include positive tone compositions wherein the exposed region is dissolved and negative tone compositions wherein the exposed region is left as a pattern. Either one is selected in accordance with the desired resist pattern, depending on ease of processing. The chemically amplified negative resist composition generally comprises a polymer which is soluble in aqueous alkaline developer, an acid generator which is decomposed to generate an acid upon exposure to radiation, and a crosslinker which forms crosslinks between polymer molecules under the catalysis of the acid to turn the polymer insoluble in the developer (sometimes, the polymer and the crosslinker are integrated together). Further a basic compound for controlling diffusion of the acid generated upon exposure is added.
Among negative resist compositions comprising the polymer which is soluble in aqueous alkaline developer, a number of negative resist compositions based on polymers using phenol units as the alkali-soluble unit were developed as best suited for the KrF excimer laser lithography. These compositions were not used in the ArF excimer laser lithography because phenol units have little or no transmittance to exposure light having a wavelength of 150 to 220 nm. Recently a highlight is drawn to these compositions again as the negative resist for the EB or EUV lithography capable of forming finer size patterns. For example, Patent Documents 1 to 3 disclose resist compositions which exhibit a very high resolution even when used in thin film form.
Beside the above-mentioned compositions, many other chemically amplified negative resist compositions have been developed. These negative working resist compositions use a crosslinker for insolubilizing the alkali-soluble polymer under the action of an acid generated upon exposure to high-energy radiation. Many crosslinkers including those disclosed in Patent Documents 1 to 3 have been developed. On the other hand, an attempt has been made to endow the polymer with the function of crosslinker. For example, it was proposed to introduce styrene units having an alkoxymethoxy group substituted thereon (Patent Document 4), recurring units having an alkoxymethylaraino group (Patent Document 5), recurring units having an epoxy group (Patent Document 6), recurring units of styrene having an acid-eliminatable group (Patent Document 7), recurring units of adamantyl having an acid-eliminatable hydroxyl group (Patent Document 8), and recurring units of aliphatic hydrocarbon and alicyclic hydrocarbon having an acid-eliminatable hydroxyl group (Patent Documents 9 to 11). Materials having an acid-eliminatable hydroxyl group are also disclosed in Non-Patent Documents 1 to 3.